1. Field of the Invention
The invention is concerned with a drive device with an eccentric gearing.
2. Description of the Related Art
In many applications of gears, a maximal drive moment is required only beginning at a particular point. Such requirements occur for example in gears which are used in a locking function. In the construction industry, this could be a window closer or, in the automotive industry, a closure tightening aid, or a gear for closing of air flaps.
If the drive occurs via an electric motor, and if a gearing with a constant transmission or speed change curve is employed, there results the disadvantage that the ratio of the transmission must be so high on the one side that the maximal moment can be achieved with the employed motor. This requires as a rule a high adjustment time or adjustment time of the gearing. The adjustment time is limited in many applications, such as for example during the closure of an air flap, which must be closed in response to fire, or in climate control devices or in window closers.
In order to maintain the adjustment time within prescribed limits, it is necessary in this case to employ a stronger and thus more expensive motor, in order to reduce the transmission ratio and therewith to maintain the adjustment time within the prescribed limits while achieving the required drive moment.
It is a further disadvantage, that in the case of stronger motors also stronger electric currents are needed, whereby also the control of the motor becomes substantially more expensive. In the design of the gear unit thus in many cases a maximum current is not to be exceeded. Even in the case of a weaker direct current motor there is the problem that the current requirement increases with decreasing motor speed.
A further disadvantage of such a design is that, in the case of the commonly employed direct current motors, the RPM of the motor depends upon the required motor moment. The motor has a changing RPM during this resetting or adjustment. This change in RPM has a negative effect on the subjective noise sensitivity and is not accepted as such in the above-described environments of usexe2x80x94in the construction and automobile industry.
It is the task of the invention to avoid the above-described disadvantages and to produce a drive device in such a manner that the required drive moment is achieved with a weaker motor and, at the same time, the adjustment time of the gearing is reduced.
The solution is comprised therein, that instead of a drive with continuous transmission, a two-stage drive is employed, wherein the second stage is an eccentric gearing. The first stage can be a conventional gearing. Preferably the first gearing stage is a harmonic drive system or a wave generator as described in detail by applicants in issued European Patent EP 0 918 961 B1 (U.S. Pat. No. 6,220,115 B1). For the purpose of disclosure the content of this patent is incorporated herein by reference. The transmission elements of the therein described wave generator (also referred to as harmonic-drive-gearing) are planetary gears.
Eccentric gears are gears with non-constant transmission behavior, and as such are already known. One example of an eccentric gear is described in DE 197 39 851 A1. Therein the gearing is used for driving a windshield wiper for an automobile. The eccentric gear is herein exclusively used in order to produce a back and forth movement of the windshield wiper. The therein described eccentric gearing is however not employed for achieving a particular transmission power curve or for reducing adjustment times.
One solution according to the invention is also concerned with a process, in which the behavior curve of the required drive moment, and the given motor characteristic curve of the employed driver, are determined for the optimal transmission curve, so that therewith the above-described disadvantages can be avoided. The optimization further distinguishes between the individual types of electric motors, for example between a direct current motor and a stepping motor.
A particular advantage of the invention is comprised therein, that by the optimization of the transmission curve during use of a direct current motor this can be driven with substantially more constant RPM over the entire range of adjustment, whereby the drive or gear noise is substantially reduced.